In the presence of incident light, photovoltaic generators produce electrical energy. Inverters are required in order to allow DC voltage produced by the photovoltaic generators to be fed into a public supply grid system. The operating point of the photovoltaic generator is continuously tracked by variation of the power output by means of a tracking device, also referred to as a maximum power point (MPP) tracker, for operational management of this inverter, such that the photovoltaic generator is operated as far as possible all the time at the point of maximum power output. An MPP tracker such as this for this purpose varies the current (I) drawn or the voltage (U) produced by the photovoltaic generator by a small amount, in each case calculates the power (P) as the product of the current and the voltage, and, possibly readjusts the current value or voltage value in the direction of higher power. The desired optimal operating point is formed by the global maximum of the PIU characteristic or the P/I characteristic.
An overview of miscellaneous known tracking methods for MPP tracking devices is provided by the article “Comparison of Photovoltaik Array Maximum Power Point Tracking Techniques”, IEEE Transactions on Energy Conversion, Vol. 22, No. 2, pages 439-449, 2007.
If a photovoltaic generator is partially shadowed, two or more local maxima typically occur in the P/U or P/I characteristic. When using MPP tracking, it is possible in this case for the operating point to be readjusted to a local maximum, which is not necessarily also the global maximum, at which the photovoltaic generator should preferably be operated. In practice, depending on the history of the occurrence of shadowing, this can lead to yield losses if the MPP tracker “winds” to a local maximum, and remains there.
In this case, it is known for the tracking mode of the MPP tracker to be interrupted at predetermined regular time intervals, and for a systematic search to be carried out for the global maximum on the P/U characteristic. By way of example, a search such as this can be based on the recording of a U/I characteristic which extends at least over the range within which the global maximum can be expected. The global maximum which is found in the search is then used as the initial operating point for the tracking mode, which is subsequently resumed. Although this does not make it possible to prevent energy losses resulting from the operating point tracking a local maximum, it does reduce them, however, since the dwell time at a local maximum is restricted to the time interval between two successive searches.
However, energy losses also occur unavoidably when searching for the global maximum since the operating point differs from the global maximum for the majority of the search time when searching for the operating point. The magnitude of these energy losses increases with the search frequency while, in contrast, the risk of potential energy losses resulting from dwelling at a local maximum falls with the search frequency. Every value for the search frequency therefore represents only a compromise, which is suitable for keeping the energy losses low only in specific incident radiation conditions.
The article “Maximum Power Point Tracking Scheme for PV Systems Operating Under Partially Shaded Conditions”, IEEE Transactions on Industrial Electronics, Vol. 55, No. 4, pages 1689-1698, 2008, discloses a method for operation of a photovoltaic generator at an operation point of maximum power wherein the power of the photovoltaic generator is monitored for sudden changes during the tracking operation. If an observed rate of change exceeds a given rate of change that is considered to be critical, a search for a global MPP is initiated. Thus, a sudden power change is seen as an indication that a search for a global MPP is required. Accordingly, the frequency of performing a search for a global MPP is highly dependent on the value of the predetermined critical rate, which itself usually is specific to the photovoltaic generator and has to be determined manually.